Elimination addition pathways

Let us conclude this discussion of the elimination-addition pathway for sulfonyl substitutions by noting some points about the behavior of sulfenes in their reactions with nucleophiles. Attack of a nucleophile Nu- on a sulfene normally occurs at sulfur to give the a-sulfonyl carbanion C—S02Nu. 

A dissociative elimination-addition pathway has also been proposed to account for the kinetics of alkaline hydrolysis of 2,4-dinitrophenyl 4 -hydroxyphenylpropionitrile in 40% (v/v) dioxane-water, although participation of the associative Bac2 mechanism cannot be ruled out since it may be facilitated by the electronic effect of the triple bond. Formation of intermediate (15), having a conjugated and cumulated double-bond system, should favour the ElcB mechanism and thereby account for the contrasting entropies of activation found for hydrolysis of (14) and the corresponding 4 -methoxyphenylpropionate. 

In 1994, it was found that several reactions thought to proceed via the vinylic SflArl mechanism were contaminated by a nonradical, a, /t-ehrninalion/addition pathway (equation 40)178. However, this elimination/addition pathway becomes inaccessible when substrates without ft (or ft1) hydrogens are utilized. Thus, the reaction of pinacolone enolate (f-Bu(CO)CH2) with l-bromo-l,2,2-triphenylethylene was touted to be the first unequivocal example of vinylic substitution exclusively by the S l pathway. 

For N-alkyl or N-aryl /3-lactams, substitution at the 4-position must involve either elimination-addition via an azetinone containing a 3.4 carbon-carbon double bond (7—>8—>9) or direct displacement by SN2 (7— 9) or SN1 (7—>10—>9) mechanisms. The elimination-addition pathway would again presumably occur by an ElcB mechanism, but with anion formation now occurring at the 3- rather than the I-position (90JOC3244). 

After free existence of l-azetin-4-one had been demonstrated, it seems that the mechanism of the so-called nucleophilic substitution reactions of 4-substituted 2-azetidinone follows an elimination-addition pathway. The intermediacy of (1) in displacement reactions is fully consistent with the stereochemistry of the reaction (83CJC1899 91TL2265). 

The aryl halide is incapable of elimination and so cannot form the benzyne intermediate necessary for substitution by the elimination-addition pathway. 

When the strong base sodium piperidide is used, reaction occurs by the elimination-addition pathway via a naphthalyne intermediate. Only one mode of elimination is possible from 1-bromo-naphthalene. 

In general, the "addition-elimination mechanism will be favored for compounds with low electron density on the beta-carbon, while the "elimination-addition" pathway will be favored by cis-isomers where there is a good chance of eliminating the elements of HX from a trans-position. 

The elimination-addition pathway proposed for the epoxidation reaction in the synthesis of flavipucine has been challenged on the grounds that this mechanism has not been established unequivocally. Sesbanine is a novel cytotoxic alkaloid occurring in the seeds of Sesbania drummondiv, its constitution (48) has been settled by spectral study and by X-ray diffraction analysis. Several amino-substituted derivatives of mimosine, (possible inhibitors of the growth of wool) have been synthesized. ... 

An X-ray diffraction analysis of ( )-flavipucine (30) has confirmed its structure and has revealed that the oxiran ring is Z in configuration. Rearrangement of the compound may be effected in several ways, best by simply refluxing in xylene (80% yield). The structure (31) of the rearrangement product has been confirmed by spectral measurements and by its reduction with zinc-acetic acid to (32), the formation of which can be rationalized, and which has u.v. spectral characteristics of a 4-hydroxy-2-pyridone rather than a 3-hydroxy-2-pyridone. These and other observations tend to confirm (31) as the rearrangement product to the exclusion of (33). " An elimination-addition pathway has been established for the epoxidation reaction employed in the synthesis of flavipucine. ... 

A convenient two-step access to valuable ethyl a-fluorocyclopropanecarboxylates that involves a Michael-initiated ring closure reaction between ethyl dichloroacetate and various terminal electron-deficient alkenes has been developed. In the second reaction step, fluorination reaction with potassium bifluoride takes place through a 1,2-elimination/addition pathway. 

The substitution reactions of R2CHP(0)(NMeR )Cl (141 I CH = fluoren-9-yl R = CHMePh) with secondary amines (Mc2NH, EtaNH, Pr NHEt) are largely stereospecific or non-stereospecific depending on the bulk of the amine and its concentration two elimination-addition pathways (Scheme 24), differing in whether... 

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